Guy Sisma-Ventura

Seawater carbonate system measurements in the Mediterranean Sea are relatively scarce, especially in its eastern basin. This study describes the monthly variations of carbonate system parameters in the open sea and coastal water column of the eastern Mediterranean Sea during the period February 2018 to January 2019. Water samples were collected for analysis of Total Alkalinity and pH _T at the shallow coastal THEMO1 and the deeper open‐water THEMO2 (1,500 m bottom depth) stations off the Mediterranean coast of Israel. In the surface layer (−1 and the pCO _2(SW) was strongly and positively correlated with temperature. During summertime, surface waters were highly super‐saturated with respect to atmospheric pCO ₂ (pCO _2(SW) ∼ 600 μatm), while during winter they were only slightly super‐saturated. The estimated net annual CO ₂ flux from the surface of the eastern Mediterranean Sea to the atmosphere during the study period was +2.8 Tg C·y ⁻¹ , which is three times greater than a previous estimate made for this region (0.9 Tg C·y ⁻¹ ), and is attributed to the smaller volume of Atlantic water input in the present study compared to the previous one as well as the substantially shallower total dissolved inorganic carbon enriched core of the Levantine Intermediate Water in this study. Based on these results, it is still expected that future warming and salinization, reduced Atlantic Water influx to the eastern Mediterranean Sea, and increased stratification will increase the flux of CO ₂ to the atmosphere in this region.

Previous studies have reported a nitrate ¹⁵ N depletion in the Mediterranean Sea compared to the global ocean, attributed to either N ₂ fixation or atmospheric deposition of anthropogenic N. In this study, we report basin‐wide full‐depth profiles of nitrate δ ¹⁵ N (vs. Air) and δ ¹⁸ O (vs. Vienna Standard Mean Ocean Water, VSMOW) in the Mediterranean Sea. Our results confirm a consistent ¹⁵ N depletion across the entire Mediterranean Sea, with significantly lower nitrate δ ¹⁵ N values in the eastern basin (2.2 ± 0.2‰) than in the western basin (2.9 ± 0.1‰). In contrast, there is no significant difference in nitrate δ ¹⁸ O between the two basins (2.2 ± 0.3‰ and 2.1 ± 0.2‰, respectively). These observations point to a supply of low‐ δ ¹⁵ N N to the Mediterranean Sea, accumulating as regenerated nitrate, which is diluted by the nitrate in the Atlantic inflow, creating an west‐to‐east gradient in nitrate δ ¹⁵ N. A four‐box model reveals that, given a water residence time of 120–170 years in the Mediterranean, a modest input rate of 1–3 Tg N yr ⁻¹ —originating from N ₂ fixation, atmospheric deposition, or their combination—is adequate to produce the observed low δ ¹⁵ N of Mediterranean nitrate. Additionally, partial degradation of dissolved organic nitrogen imported from the Atlantic may add low‐ δ ¹⁵ N nitrate to the Mediterranean, but it alone cannot explain the full isotopic signal. Distinguishing among these sources will be aided by the reconstruction of Mediterranean nitrate δ ¹⁵ N through time using either time‐series data of nitrate δ ¹⁵ N or calcareous fossil‐bound organic nitrogen isotope ratios.

Estimates of primary productivity have traditionally disregarded dark inorganic carbon fixation by marine microorganisms. Currently, only limited data are available from different systems on this potentially ecologically important process. We present monthly dark inorganic carbon fixation and photosynthetic rates from the euphotic layer of the northern Gulf of Aqaba collected over a decade between 2010 and 2020. Averaged dark inorganic carbon fixation rates from surface to 100 m depth, ranged from 99 to 173 mg C m⁻² d⁻¹, which corresponds to ~ 43% of the annual primary productivity at this location. The lowest dark inorganic carbon fixation rates were found during winter, contributing ~ 7.5% of the integrated primary productivity. During the oligotrophic summer, dark inorganic carbon fixation comprised a larger fraction of the integrated primary productivity estimated as ~ 12%. In accordance, dark inorganic carbon fixation contributed ~ 6% to the particulate organic carbon flux during the winter and ~ 30% during summertime. Complimentary nutrient‐enrichment bioassays of seawater from 5 m show that dissolved organic nutrient enrichment (P and C based) significantly elevates dark inorganic carbon fixation, whereas addition of dissolved inorganic nutrients (, , or both) significantly increased photosynthesis but to a lesser extent dark inorganic carbon fixation. These results suggest that dark inorganic carbon fixation may be an important biochemical process throughout the euphotic zone of oligotrophic seas, and thus should be incorporated into oceanic carbon production estimates.

Discharge of gas-rich brines fuels productive chemosynthetic ecosystems in the deep sea. In these salty, methanic and sulfidic brines, microbial communities adapt to specific niches along the physicochemical gradients. However, the molecular mechanisms that underpin these adaptations are not fully known. Using metagenomics, we investigated the dense (∼10 ⁶ cell ml- ¹ ) microbial communities that occupy small deep-sea brine pools found in the Southeastern Mediterranean Sea (1150 m water depth, ∼22°C, ∼60 PSU salinity, sulfide, methane, ammonia reaching millimolar levels, and oxygen usually depleted), reaching high productivity rates of 685 µg C L ^-1 d ^-1 ex-situ. We curated 266 metagenome-assembled genomes of bacteria and archaea from the several pools and adjacent sediment-water interface, highlighting the dominance of a single Sulfurimonas , which likely fuels its autotrophy using sulfide oxidation or inorganic sulfur disproportionation. This lineage may be dominant in its niche due to genome streamlining, limiting its metabolic repertoire, particularly by using a single variant of sulfide: quinone oxidoreductase. These primary producers co-exist with ANME-2c archaea that catalyze the anaerobic oxidation of methane. Other lineages can degrade the necromass aerobically ( Halomonas and Alcanivorax ), or anaerobically through fermentation of macromolecules (e.g., Caldatribacteriota, Bipolaricaulia, Chloroflexota, etc). These low-abundance organisms likely support the autotrophs, providing energy-rich H ₂ , and vital organics such as vitamin B12.